Spinal cord stimulation with interferential current

ABSTRACT

A stimulator and a method for the treatment of intractable pain syndromes by electrical stimulation of the spinal cord is disclosed in which implantable electrodes positioned around a targeted area of the spinal cord transmit an interferential current that has a base medium frequency alternating current between 500 Hz-20 KHz. A digital signal processor generates a sine-wave-like waveform from a pulse generator which after further processing is used to generate at least two circuits for use in producing the beat frequency signal. An effective area of stimulation is controlled by the quantity of electrodes, positioning of the electrodes and electrode cross pattern orientation. Amplitude modulation of electrical circuits created at the electrode placements also augments the effective area of stimulation. The stimulator and method reduce accommodation of the body to the electrical stimulation and provide deeper penetration of the resultant signal.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 15/076,834 filed on Mar. 22, 2016, which claims priority to U.S.patent application Ser. No. 14/606,405 filed on Jan. 27, 2015, whichclaims priority to U.S. patent application Ser. No. 10/761,424 filed onJan. 22, 2004, which claims the benefit of U.S. Provisional PatentApplication No. 60/441,326 filed Jan. 22, 2003, all of which are herebyincorporated by reference in their entirety into the present disclosure.

FIELD OF THE INVENTION

The present invention is generally related to spinal cord stimulationand, more particularly, is related to an apparatus and method for theelectrical stimulation of the spinal cord using an interferentialcurrent pattern for treating chronic pain conditions.

BACKGROUND OF THE INVENTION

Electrical stimulation of the posterior spinal cord, spinal cordstimulation (SCS), has developed into an effective therapeutic tool fortreating chronic pain conditions. However, very little is known aboutthe sites of activation or the neural mechanisms evoked by SCS thatrelieve pain and promote changes in the function of somatic and visceralstructures.

Spinal Cord Stimulation is most commonly used for patients with chronicintractable pain syndromes. It has also been useful for treatingmovement disorders and is occasionally used following head injuries.However, one complication with SCS is that of accommodation orhabituation to the stimulation signal. Companies that manufacture spinalstimulation devices have developed complex stimulation programs anddevoted chapters on techniques to reduce the problem of accommodationduring SCS (Alfano S, Darwin J, Picullel B: Spinal Cord Stimulation,Patient Management Guidelines for Clinicians, Medtronic, Inc.).Accommodation is when the body habituates or becomes accustomed to anactivity or signal and then starts to ignore or ‘tune it out’. Byvarying the signal or keeping the focal point of the signal moving,accommodation can be minimized. The concept of using interferentialstimulation with implantable leads to decrease the problem ofaccommodation might prove to be advantageous.

Dorsal Column Stimulation (DCS) or SCS using an electricalinterferential current pattern has shown to be a cost benefit intreating chronic pain disorders in patients (Dorsal column stimulation:cost to benefit analysis; Acta Neurochir Suppl (Wien), 52( )0: 121-3,1991).

SCS stimulates the dorsal column in a somewhat superficial manner aspointed out by Holsheimer (Holsheimer J: Which Neuronal Elements areactivated Directly by Spinal Cord Stimulation, Neuromodulation, Volume5, Number 1: 25-31, 2002). The electrodes are normally attached to thedura matter in the epidural space, and most of the current distributionremains in the cerebrospinal fluid (CSF) and does not project deeplyinto the dorsal column Providing an interferential component to theelectrode array of the SCS allows the crossing of the two signalswherein the resultant additive effect of the beat frequency producesdeeper penetration of the signal and a higher resultant amplitude at thestimulation site. The interferential current would recruit largernumbers of dorsal column fibers and provide greater levels of painrelief and benefit to intractable pain patients.

Thus, a heretofore unaddressed need exists in the industry to addressthe aforementioned deficiencies and inadequacies with regard toaccommodation or habituation to the spinal cord stimulation signal whenused in the treatment of chronic pain syndromes.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an apparatus and method forthe treatment of chronic pain syndromes using electrical stimulation ofthe spinal cord. The present invention utilizes an interferentialcurrent that has a base medium frequency alternating current between 500Hz and 20 KHz. An interferential current is set up between two circuitsthat are arranged in a cross-pattern on the subject's targeted area ofstimulation. Where the circuits superimpose in a cross-pattern, theresultant beat frequency will be the difference between the frequenciesof the two circuits and the amplitude will be additive and greater thaneither circuit alone. The range of the beat frequency is usually between1-250 Hz. Multiple levels of stimulation can be treated depending uponthe electrode placement, pairing and modulation pattern selected. Therange of output would be from 0-11 volts per circuit depending on thepatient's needs and the pulse width is commonly set at 210 microsecondsbut it could range from 10-600 microseconds. The amplitude can bemodulated in the respective circuits to increase the area of targetedstimulation. This type of current (Interferential) provides improveddirectional control, decreased accommodation/habituation and increaseddepth of penetration in comparison to other standard implantablestimulation systems and their accompanying surgical leads. Theamplitudes of the outputs in the respective circuits may be modulated toincrease the area of targeted stimulation. Interferential current allowsimproved directional control and depth of penetration in comparison toother stimulation techniques.

Briefly described, in architecture, one embodiment of the invention,among others, can be implemented as follows.

Digital signal processors (DSPs) are used for improving the accuracy andreliability of digital signals that are used extensively in thecommunications field. Digital signal processing works by standardizingor clarifying the output of a digital signal. In this embodiment, thedigital signal processor is used to shape multiple pulsatile waveformsto approximate the output of a sine-wave generator. In anotherembodiment of the invention, the digital signal processor is replacedwith a field-programmable gate array (FPGA). An FPGA is an integratedcircuit that can be programmed in the field after it is manufactured andtherefore allows users to adjust the circuit output as the needs change.Both the DSP and the FPGA process a digital signal into apseudo-sine-wave current waveform from the digital pulses generated by apulse generator. The pseudo-sine-wave current waveform is transmittedthrough implantable quadripolar leads with eight electrodes at atargeted area creating a pair of interferential currents.

Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a perspective view of an interferential current set up by twocircuits that are arranged in a cross pattern;

FIG. 2 is a perspective view of an interferential current patternindicating the current intensity level and area of beat frequencyformation;

FIG. 3 is a perspective view illustrating the effective area ofstimulation resulting from the crossing of separate circuits; and

FIG. 4 is a diagram illustrating interferential stimulation using twoimplantable quadripolar leads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention and modifications thereof willnow be described with reference to the drawings.

FIG. 1 shows a stimulator 100 for the electrical stimulation of thespinal cord utilizing an interferential current 110 that has a basemedium frequency alternating current within the range of 500 Hz-20 KHz.The interferential current 110 is set up between two circuits 118, 120that are arranged in a cross-pattern. A first pair of implantableelectrodes 108, 208 are positioned on a subject's spinal column 112,preferably the dorsal column, at one set of diagonal corners of atargeted area 214 (see FIG. 2). A second pair of implantable electrodes108, 208 is then positioned at the other set of diagonal corners of thetargeted area 214. Preferably, the electrodes 108 are attached to thedura matter in the epidural space. A digital signal processor 102 isconnected to the first and second pairs of surface electrodes 108. Whena signal generating source 104 is connected to the digital signalprocessor 102, a sine-wave-like waveform signal output 106 is created.The digital signal processor 102 improves the accuracy and reliabilityof digital signals. The digital signal processor 102 processes themultiple pulses 116 from the signal generating source 104 to approximatea sine-wave (pseudo-sine-wave or sine-wave-like). Thus, that type ofcurrent recruits larger numbers of dorsal column fibers and providesgreater levels of pain relief.

The digital signal processor 102 generates individual pulses 106 ofdiffering widths and resultant amplitudes. Preferably, the pulse widthis set at 210 microseconds, but can range from 50-600 microseconds. Whenthose differing pulses 106 are driven into a transformer (not shown),the pseudo-sine-wave is produced. A pulse generator 104 is connected tothe digital signal processor 102 and supplies a pulsed digital signaloutput 116 to the digital signal processor 102. The digital signal 106processed by the digital signal processor 102 creates a first circuit118 and a second circuit 120 at the first and second pairs of surfaceelectrodes 108, 208, respectively. Preferably, the range of output ofthe electrical circuits 118, 120 are 0-11 volts per circuit, dependingon the patient's needs for pain treatment. Where the first and secondcircuits 118, 120 superimpose (cross), the resultant beat frequency(which is preferably between 1 and 250 beats/second) will be thedifference between the frequencies of the two circuits, and theamplitude will be additive and greater than either circuit alone (FIG.2).

Multiple target areas of the spinal cord can be treated depending uponthe quantity and placement of the first and second pairs of electrodes308, and by modulating the amplitudes of the outputs of the first andsecond circuits 318, 320 (see FIG. 3). Modulating the outputs of thefirst and second circuits 318, 320 increases the area of the targetedstimulation. The depth of modulation can vary from 0 to 100% and dependson the direction of the currents established by the first and secondcircuits 318, 320. It has been shown that when the first and secondcircuits 318, 320 intersect at 90°, the maximum resultant amplitude andthe deepest level of modulation is half-way between the two circuits(45° diagonally). (See FIG. 2). Hence, the target area of stimulationcan be augmented by modulation of the amplitudes of the outputs of thetwo circuits.

FIG. 4 illustrates two interferential currents 406 with sine-wave-likewaveforms that are produced by two implantable quadripolar leads 409.Each quadripolar lead 40 includes four electrodes 408 for a total ofeight. The two quadripolar leads 409 allow a greater target treatmentstimulation area of the spinal cord. However, the invention could alsoapply to the use of two bipolar or octapolar lead systems, and othersuitable devices. The electrodes could be activated in variouscombinations and patterns, and not just as shown in the drawings.

A field-programmable gate array (not shown) can also be used to shapemultiple pulsatile waveforms to approximate the output of a sine-wavegenerator instead of the digital signal processor 102 described above.The FPGA is an integrated circuit that can be programmed in the fieldafter it is manufactured and allows its user to adjust the circuitoutput as desired. In an alternative embodiment, the digital signalprocessor may be replaced with the FPGA. Whereas DSP processorstypically have only eight dedicated multipliers at their disposal, ahigher end FPGA device can offer up to 224 dedicated multipliers plusadditional logic element-based multipliers as needed. That allows forcomplex digital signal processing applications such as finite impulseresponse filters, forward error correction, modulation-demodulation,encryption and applications such as utilized in the present invention.

It should be emphasized that the above-described embodiments of thepresent invention, particularly, any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding on the principles of the invention. Many variations andmodifications may be made to the above-described embodiment(s) of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

I claim:
 1. A method for spinal cord stimulation treatment for treatingpain using electrical stimulation of the spinal cord, the methodcomprising: positioning a first pair of implantable electrodes to a duramatter in an epidural space proximate to a subject's spinal cord atpredetermined locations; positioning a second pair of implantableelectrodes to the dura matter in the epidural space proximate to thesubject's spinal cord at predetermined locations; and transmittingsignals of first and second frequencies through the first and secondpairs of implantable electrodes respectively, so that the signals of thefirst and second frequencies interfere with each other to produce atleast one beat signal proximate to the subject's spinal cord using acurrent with a base medium frequency of at least 500 Hz but no more than20 KHz.
 2. The method of claim 1, further comprising: positioning thefirst pair of implantable electrodes and the second pair of implantableelectrodes so that a first circuit is created between a first electrodeof the first pair of implantable electrodes and a first electrode of thesecond pair of implantable electrodes and so that a second circuit iscreated between a second electrode of the first pair of implantableelectrodes and a second electrode of the second pair of implantableelectrodes, wherein the first circuit and the second circuit arearranged to cross one another.
 3. The method of claim 1, furthercomprising: supplying digital signal pulses to a digital signalprocessor via a pulse generator; the digital signal processor processingthe digital signal pulses to approximate a sine-wave-like outputwaveform; and transmitting the sine-wave-like output waveform as signalsof the first and second frequencies through the first and second pairsof implantable electrodes.
 4. The method of claim 1, further comprisingmodulating amplitudes of outputs of the first and second pairs ofimplantable electrodes.
 5. The method of claim 1, wherein the methodincludes transmitting signals using a current with a resultant beatfrequency of no more than 250 Hz.
 6. The method of claim 1, furthercomprising: altering a targeted area of the subject's spinal cord bymodulating amplitudes of the signals of first and second frequencies. 7.The method of claim 2, further comprising varying a depth of modulationof the at least one beat signal based on an angle of crossing the firstand second circuits.
 8. The method of claim 2, wherein a resultingmodulation of the at least one beat signal is a deepest level ofmodulation at about 45° between the first and second circuits.
 9. Themethod of claim 1, further comprising varying locations of the first andsecond pairs of implantable electrodes along the spinal cord.
 10. Themethod of claim 1, wherein the method includes transmitting signalsusing a current with a voltage output of 11 volts maximum.
 11. Themethod of claim 1, wherein the method includes transmitting signalsusing a current with a pulse width of 210 microseconds.
 12. The methodof claim 1, wherein the method includes transmitting signals using acurrent with a pulse width comprising a range of at least 10microseconds but no more than 600 microseconds.
 13. The method of claim1, wherein the first pair of implantable electrodes and the second pairof implantable electrodes are included on two quadripolar leads.